The present invention concerns a method for cleaning heat exchangers. More specifically, the present invention concerns an in-situ method for cleaning a feed-effluent heat exchanger in a system for stripping sour water to purify the water.
Water and steam are used extensively in petroleum refining operations. During use, the water and steam often come in contact with ammonia, cyanide and so-called "acid gases" such as hydrogen sulfide and carbon dioxide. Such contaminant materials dissolve fairly readily in water, and in condensing steam, and refinery processing water streams thereby often becomes seriously contaminated. The contaminated water is commonly called "sour water".
Sour water is usually unsuitable for use in refining operations or in steam generations, and is considered a waste material. It is not desirable to discharge large amounts of sour water into normal waste disposal systems because of the high pollutant content. Therefore, various purifications systems are conventionally employed to remove the pollutants and permit the water to be used further in refining operations.
A common conventional method for removing the contaminants from sour water is by treating the contaminated water in a sour water stripper. Typically, a sour water stripper includes means for stripping the water, means for storing the sour water feed, and a feed-effluent heat exchanger which is employed to minimize the heat input necessary for carrying out the stripping operation. Stripping means employed in conventional systems usually include a distillation column, in which the sour water is passed downwardly in countercurrent flow to a hot gas, such as steam, flue gas or natural gas. The sour water storage means employed in most operations is typically a sump or surge vessel to insure a steady supply of feed to the stripper. Not uncommonly, the sour water storage facility may be a large capacity storage vessel, in which several days' supply of water can be stored if necessary. Of course, the stripping system normally includes other conventional components, such as an overhead condensing system for treating the steam and contaminants removed overhead from the stripping column, pumping means, controlling means, etc., and the stripping column is normally equipped with internals to facilitate contact between the stripping gas and water, such as trays, baffles, or the like. In a steam-stripping system, sour water enters the top of the distillation column and stripped, purified water is removed from the bottom of the column. Steam enters the bottom of the column and steam, hydrogen sulfide and ammonia, etc., are removed from the top of the column. The steam is condensed and the hydrogen sulfide and ammonia are recovered as gases for further treatment. Since the condensed steam is usually contaminated with a substantial amount of hydrogen sulfide and ammonia, it is usually returned to the sour water storage facilities for recycling to the stripper.
In a feed-effluent heat exchanger, the hot stripped water effluent recovered from the bottom of the stripping column is heat exchanged with the cooler sour water feed, which is then fed to the top of the column. It has been found that the stripped water pathway in the feed-effluent exchanger can become seriously and rapidly fouled during the heat exchanging operation, typically due to the deposition of elemental sulfur on the surfaces in the heat exchanger. Cleaning the stripped water side of a feed effluent exchanger by conventional mechanical means is a tedious and time-consuming procedure. The present invention is directed, in part, at providing a means for cleaning the stripped water pathway in a feed-effluent heat exchanger without the use of extraneous mechanical equipment, expensive chemical cleaners or the like.